The present invention relates generally to semiconductor structures and more particularly to semiconductor laser structures having quantum wires fabricated employing in situ photo induced modifications to compound semiconductor films during epitaxial growth.
The employment of quantum wires to confine the active regions of semiconductor lasers to two dimensions in quantum well regions is known in the art. Quantum wires in semiconductors are regions in which the charge carriers are quantum confined in the 2 dimensions orthogonal to the axis of the wire. Quantum effects in GaAs layers are most strong when the thickness is less than 50 nm. A quantum wire will still be useful if only one of its dimensions is less than 50 nm. For GaAs this typically means a region for the quantum wire in a semiconductor smaller than about 50 nm by 100 nm.
In situ fabrication of arrays of quantum wires has been contemplated and demonstrated by Fukui et al., "(AlAs).sub.0.5 (GaAs).sub.0.5 fractional-layer superlattices grown on (001) vicinal surfaces by metalorganic chemical vapor deposition" in Appl. Phys. Letters 50,824 (1987) and Tsuchiya et al. in Phys Rev Letters 62,466 (1989) using stepped surfaces obtained with off axis substrates to produce tilted superlattices. A difficulty with this approach is the propagation of this surface topography through the relatively thick layers required for the cladding layers of a semiconductor laser structure.
A quantum well layer such as GaAs has been grown to fill in a previously etched V-shaped groove in the substrate to form a laser structure. E. Kapon et al., "Quantum Well Lasers Using Patterned Growth", 1988 IEEE Lasers and Electro-Optics Society Annual Meeting, Paper OE-2, Kapon et al., "Patterned quantum well semiconductor injection laser grown by molecular beam epitaxy", Appl. Phys. Letters 52,607 (1988).
The resulting quantum well layer is thinner on the sides of the groove than near the vertex but the thin sidewall quantum layer is still present. As a result, carrier recombination and lasing in this structure may occur in the quantum well regions on the sides of the groove since these regions are larger and therefore provide more gain than the smaller vertex region. Our invention addresses the elimination of the sidewall growth.
What is desired is a process, particularly as implemented in molecular beam epitaxy (MBE) or metalorganic chemical vapor deposition (MOCVD), wherein ultrafine patterning of ultrathin layers can be achieved in situ without growth interruption by some off-line or nongrowth procedure or process.
There are two examples known to us where patterning may be achieved by quasi-in situ thermal processing wherein thermal etching is employed to selectively remove GaAs. In one example, a n-GaAs layer over a p-AlGaAs layer is first, selectively chemically etched in a particular region followed by thermal etching to remove the remaining thin GaAs left from chemical etching before proceeding with regrowth of the p-AlGaAs layer. This forms a buried reverse biased current confinement mechanism in a double heterostructure laser. H. Tanaka et al, "Single-Longitudinal-Mode Self Aligned AlGa(As) Double-Heterostructure Lasers Fabricated by Molecular Beam Epitaxy", Japanese Journal of Applied Physics, Vol. 24, pp. L89-L90, 1985.
In the other example, a GaAs/AlGaAs heterostructure partially masked by a metallic film is thermally etched in an anisotropic manner illustrating submicron capabilities for device fabrication. A. C. Warren et al, "Masked, Anisotropic Thermal Etching and Regrowth for In Situ Patterning of Compound Semiconductors", Applied Physics Letters, Vol. 51(22), pp. 1818-1820, Nov. 30, 1987. In both of these examples, AlGaAs masking layers are recognized as an etch stop to provide for the desired geometric configuration in thermally etched GaAs, although it is also known that, given the proper desorption parameters, AlGaAs may also be thermally etched at higher temperatures with different attending ambient conditions vis a vis GaAs.
However, none of these techniques employ in situ photo induced evaporation as a technique in a film deposition system to incrementally reduce, on a minute scale, film thickness in patterned or selective locations at the growth surface either during or after film growth, producing smooth sculptured surface morphology which is a principal objective of this invention.
It is an object of this invention to bring about in situ removal or desorption of selected surface regions or layers of compound semiconductors employing induced evaporation enhancement in metalorganic chemical vapor deposition (MOCVD) epitaxy and to apply this method in the fabrication of semiconductor laser structures with active layers containing in situ fabricated quantum wire.
It is another object of this invention to fabricate a quantum wire layer in a predetermined groove of a semiconductor laser structure and to eliminate the side regions of a deposited quantum well layer in the groove and adjust the lateral dimensions of the quantum well layer to form the quantum wire.
It is an object of this invention to fabricate multiple quantum wire layers, both horizontally or vertically, or in a two-dimensional array of a semiconductor laser structure.